import java.util.Scanner;

/**
 * Created with IntelliJ IDEA
 * Description:
 * User: Administrator
 * Data: 2023 - 04 - 18
 * Time: 13:20
 */
//找单身狗
public class test4 {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int[] arr1 = new int[11];
        for (int i = 0; i < arr1.length; i++) {
            arr1[i] = scanner.nextInt();
        }
        int ret = 0;
        for (int j = 0; j < arr1.length; j++) {
            ret ^= arr1[j];
        }
        System.out.println(ret);
    }
}
